Chemical brightening and electrolytic polishing of aluminum



2 5 2 1 O Q QQ PEECQFNCF W NER Jan. 3, 1961 A. CYBRIWSKY ETAL ,9

CHEMICAL BRIGHTENING AND ELECTROLYTIC POLISHING or ALUMINUM Filed March1, 1954 '7, Total oier By Volume lie '0 |o.9

I, Toiol Woier 8y Weight g 5.55 5 3 61 52a Volume I. Commericel L84Sulphuric Acid Water Free Add By Weight '3 e 80 DU a "/e Woier FIZOGAcid By Volume 65 "WW-Y v w ,Volume 'lo Gommericoi I."2 Phosphoric Acid25 M 5 %Woier Freq: Acid By Weighi IJ g Water Free Acul By Volume IO i gV l Commeri :01 L84 Suipi' uric Acid By Volume At oom Temp. 72.5 7 577.5 8 8 2.5 o I 1 i I l I i i O 27.5 25 22.5 20 I25 I5 I. CommericolL72 Phosphoric Acid ByVolume At Room Temp. INVENTORS ALEXANDER CYBRIWSKYNICHOLAS MOSTOVYCH WWW ATTORNEY United States Patent CHEMICALBRIGHTENING AND ELECTROLYTIC POLISHING 0F ALUMINUM Alexander Cybriwslryand Nicholas Mostovych, Louisville, Ky., assignors to Reynolds MetalsCompany, Louisville, Ky., a corporation of Delaware Filed Mar. 1, 1954,Ser. No. 413,062

12 Claims. (Cl. 204-1405) The desirability of providing a bright surfacefinish on aluminum has long been recognized. Various types of chemicaland electrolytic treatments have heretofore been proposed, but, so faras we have been able to determine, none has yet succeeded in producingconsistently good results on aluminum and the alloys thereof.Furthermore, the brightening of aluminum by successive chemicalbrightening and electrolytic polishing operations, has always heretoforerequired, so far as we know, separate chemical brightening andelectrolytic polishing baths of entirely dilferent character Theprincipal object of this invention is to provide a chemical bath whichis composed of relatively inexpensive ingredients, is substantiallynon-fuming when out of con tact with aluminum, is stable so that it isreadily maintained at proper strength and composition, and is capable ofbeing used either as a brightening bath or as an electrolytic polishingbath or as both.

Another important object is to provide a chemical bath which, when usedas a chemical brightening bath or as an electrolytic polishing bath oras a combination chemical brightening and electrolytic polishing bath,gives extremely good surface improvement results on aluminum and on awide range of its alloys.

Further objects are to provide a novel chemical brightening process, anovel electrolytic polishing process and a novel combination process.

Our invention resides first in formulating a novel bath having, as itsprimary ingredients, relatively large and small quantities of commercialsulfuric and phosphoric acids respectively and in discovering that abath of this character may be used as a chemical brightening or as anelectrolytic polishing bath or as a combination chemical brightening andelectrolytic polishing bath.

The figure is a graph showing the range of acid mixtures in bathsformulated in accordance with our invention and the amount of water ineach of said mixtures.

For the sake of clarity, the three processes utilizing one and the samebath are delt with first.

CHEMICAL BRIGHTENING PROCESS The steps of our chemical brighteningprocess may be outlined as follows:

( 1) Provide a clean or dirty aluminum article;

(2) Dip the article into a hot chemical bath, formulated in accordancewith our invention;

(3) Remove the article from the bath and allow to cool;

(4) Rinse the article in water; and

(5) Apply a protective film to the article.

The aluminum, provided for treatment in accordance with this invention,should be clean but it may be dirty provided the dirt is of a charactersuch that it comes olf easily. Thus aluminum coming from a fresh buttingoperation may be used without any cleaning operation since fresh buffingdirt comes off very readily in the chemical bath. Where aluminum hasbeen buffed and allowed to stand, the bulfing dirt tends to stick to thealuminum and 2,967,136 Patented Jan. 3, 1961 this will interfere withthe practice of this invention unless the dirt film is removed.

The aluminum is dipped into a chemical bath, formulated in accordancewith our invention as hereinafter described. It remains in that bath fora period of time ranging from one minute, if the surface of the aluminumis smooth, up to three to five minutes or more if the surface of thealuminum is rough. Mild agitation of the bath is desirable. This can beaccomplished by gently moving the parts to be brightened or by air. Thechemical bath temperature should be maintained between 230 and 300 F.(HO-148.9 C.) but the preferred temperature range is between 250 and 260F. (121.1-126.4 C.). When the aluminum is removed from this bath, itwill be covered by a layer of viscous material ranging in color fromcreamy white to creamy white yellow. This layer should not be removedbefore the rinsing step since it tends to prevent any etching of thealuminum.

When the layer-protected aluminum is removed from the bath, it should beallowed to stand for a period sufficient to permit it to drain and cool.This period ranges from ten seconds upwardly but it need not be of greatduration. Ordinarily ten to twenty or thirty seconds will be enough.But, when desirable or necessary, it may be allowed to stand for longerperiods.

The aluminum is rinsed or washed in a bath of warm or cool tap water,room temperature being preferable. The rinsing or washing actionnormally will take thirty to sixty seconds and should be vigorous enoughto insure the removal not only of any free acids but also of thecompounds which may have been formed or deposited on the aluminum suchas sulfur, aluminum sulfate, and aluminum phosphate, etc. The rinsewater preferably is agitated or circulated as by flowing it continuouslyfrom a tap through the rinsing bath to an overflow leading to the sewer.There is apparently no brightening action after it is remvoed from thebrightening bath. If the work is cooled before rinsing, there will be noetching action during the rinsing step. Etching is usually evidenced bywhitish spots. The occurrence of etching, during rinsing, appears to bea function of the accumulation or concentration of acid in the rinsingbath and the temperature of that bath. As the acid concentration or thetemperature of the rinsing bath increases, the tendency to etchincreases. The rinsing step may be repeated one or more times ifdesired.

The aluminum may now be dried. Preferably a protective film is appliedthereto either by conventional anodizing before drying or by lacqueringafter drying.

ELECTROLYTIC POLISHING PROCESS This process is identical with thechemical brightening process except for the dipping step 2 and thetransfer step 3. In the electrolytic dipping step, the aluminum isdipped into the chemical bath, which new functions as an electrolyticbath, for a period of time ranging from thirty seconds to two minutes.The aluminum functions as the anode of the chemical-electrolytic bathand a current flow should be maintained in that bath with a currentdensity ranging, in amperes per square foot of surface area of aluminum,from 40 with a low water content in the bath to with a high watercontent, the bath voltage being from 8 to 15 volts. The bath temperatureranges from 200 to 250 F.

Upon removal from the chemical-electrolytic bath, the aluminum should betransferred to the rinsing bath immediately or promptly to avoidetching. This is due to the fact that the protective layer, which isobtained in the chemical brightening process, is either eliminated orsubstantially reduced by the electrolytic polishing process. Coolingbefore rinsing is not necessary or even desirable. The flow of bathwater should be such as to COMBINATION PROCESS The steps of this processare the same as the electrolytic polishing process except for step 2. Instep 2 the aluminum is dipped into the chemical bath for chemicalbrightening purposes as in the brightening process above outlined andunder the same conditions except that the temperature of the bath is nowmaintained in the range of 230 to 250 F. Thereupon the current is turnedon in that bath to practice step 2 of the electrolytic process. Thearticle may, of course, be transferred to a separate electrolyticpolishing bath which may he, and preferably is, of the same compositionas the chemical brightening bath, but, in this event, the temperatureranges may be 230300 F., for brightening and ZOO-250 F. for polishing.If transferred, the transfer time between the chemical brightening bathand the electro polishing bath is unimportant because of the protectivelayer produced during chemical brightening. After the electrolyticpolishing step, rinsing should follow immediately as before.

COMPOSITION OF BATHS While our chemical brightening bath is maintainedat a temperature ranging from 230 F. to 300 F. and preferably between250 F. and 260 F., its room temperature composition essentiallycomprises:

(1) Commercial phosphoric acid ranging, in volume,

from 15% up to 30%, 25% being preferred;

(2) Commercial sulfuric acid ranging, in volume, from 85% down to 70%,75% being preferred; and

(3) Silver salt ranging, by weight, from 0.001% to The room temperaturecomposition of our combination bath is the same but, in our electrolyticpolishing bath, the silver salt may be omitted.

The percentage values given above are approximate; hence should be readas not substantially below the lower or above the higher limits stated.

The various oxyacids of phosphorus may be regarded as derived,theoretically at least, by hydration of the various oxides ofphosphorus. Some of the oxyacids of phosphorus, derived from loweroxidation states of phosphorus, decompose upon heating with theresultant formation of the higher oxyacids of phosphorus, and, wheresuch decomposition is not objectionable, they may be used in ourcompositions. We prefer to employ orthophosphoric acid, butpyrophosphoric acid and metaphosphoric acid may be employed. Theoxyacids of phosphorus may be employed as the soluble acid salts inwhole or in part. For example, in place of ortho phosphoric acid, themono alkali metal phosphate may be employed. Other alkali metal salts ofortho-phosphoric acid may be employed provided enough excess sulfuricacid is present to avoid the depletion of sulfuric acid in the bath. Thewater soluble salts used should not be those which will react in thebath to form insoluble compounds.

While the silver salt is mentioned above as an essential ingredient inour brightening and combination baths it is not essential therein whentreating the following wrought aluminum and/or wrought alloys; (1) highpurity aluminum 99.80% Al, (2) commercially pure aluminum (2S alloy),(3) aluminum alloys containing magnesium silicide as the principalhardening agent (63S alloy with a nominal content of 0.70% magnesium and0.45% silicon and 61S alloy with a nominal content of 1.00% magnesiumand 0.60% silicon), (4) aluminum alloy containing 1.25% manganese (3Salloy) and (5) aluminum alloys containing less than 0.5% magnesium asthe principal hardening agent. All percentages herein given are byweight.

However the addition of a silver salt makes it possible treatingaluminum alloys containing magnesium from 0.5 to 5.0% by weight. Anexample of an alloy would be C57S which contains a nominal magnesiumcontent of 1.0%. Since the composition of aluminum articles to betreated may vary from day to day in many commercial operations, thesilver salts should be and preferably are incorporated in the bath forall treatments. These salts may be silver nitrite, silver nitrate orsilver cyanide or their equivalents.

THE FIGURE The figure, for clarity, is based on the assumption that thecombined volumes of commercial phosphoric and sulfuric acids make up100% of the bath volume; hence the silver salt addition is based on theweight of the commercial acid contents of the bath as a whole. Thefigure also assumes that the specific gravities of commercial sulfuricand phosphoric acids are 1.84 and 1.72 respectively and it gives thecommercial acid contents of the bath in sulfuric to phosphoric volumeratio form ranging from 70/30 to /15. This figure shows, for each volumepercentage of commerical acid employed, at room temperature, thecorresponding volume and weight percentage of each acid on a water-freebasis and the corresponding volume and weight percentage of the totalwater content in the bath.

Furthermore it will be understood that the difierences between thevolume percentage of commercial acid and the corresponding volume andweight percentages of the same acid on a water-free basis, will give thewater content of each acid in terms of both volume and weightpercentages. But all of these percentages obtain under room temperatureconditions which are assumed to be 72 F.

WATER CONTENT Under operating conditions, at temperatures above 212 F.,the water content will be lower than that shown in Figure 1 since waterwill boil ofi until an equilibrium condition is reached depending uponthe operating temperature employed and the humidity of the atmosphere.This reduction in water content is very desirable with smooth surfacedaluminum because the lower the water content, the better and moreconsistent are the results. The addition of water to the bath is notprohibited, but, it should be avoided since it increases the tendency ofaluminum in each bath to etch rather than to brighten and increases thecurrent flow in the electrolytic bath. Accordingly the recommendedprocedure is to bring the bath up to the operating temperature andpreferably maintain it.at that temperature until the water contentstabilizes before instituting the regular use of the bath.

With rough surfaced aluminum, a higher water content 'is preferred but,when used, it must be carefully controlled because of the increasedtendency to etch.

BUFFIN G TREATMENT Bufiing of an article before it is either brightenedor polished or both, is usually not necessary because our brightening orpolishing bath has a tendency to level off shallow scratches and thedrawing marks characteristic of shallow drawing operations. Even deepscratches can be removed with a longer dipping time but, in someinstances at least, it will be more desirable to butf out deepscratches.

TEMPERATURE sitive. Where the bath is too sensitive, the first part ofan article entering the bath will be overtreated if the last partentering the bath receives the proper treatment. Overtreatment geneallyresults in a surface which may be described as an etched surface.

BATH RATIO AND MAINTENANCE In the operation of the bath, both phosphoricand sulfuric acid are lost through chemical reaction. Since the sulfuricacid loss proceeds at a rate faster than the phosphoric acid loss, theprogressively higher sulfuric to phosphoric acid ratios enable the bathto be operated over progressively longer periods of time beforereplenishmcnt is required. This ratio, however, should not be permittedsubstantially to exceed the top limit of 85/15 because there is aprogressive increase in the tendency of the bath to etch the aluminum.Neither should it be permitted to fall substantially below the lowerlimit of 70/30 because of the progressive decrease in the production ofgood results. Thus, as the ratio closely approaches 65/35, the resultsbecome too inconsistent to permit a satisfactory continuance of eitherthe brightening or the polishing operation or both. Accordingly, whenthe bath becomes depleted, it may be restored by titrating for each acidand then adding the requisite quantities required to bring the bath upto the desired acid ratio.

REACTION FUMES We are unable to explain completely, the mechanism of thereactions occurring in either or both operations. When heated but not inuse, the bath does not produce fumes or odors to any appreciable extent.When aluminum is immersed in the hot bath, there is a noticeable odor ofsulfur dioxide and hydrogen sulfide and some sulfur formation, whichindicates the sulfuric acid is being reduced. However, sulfur does notsettle out to any substantial extent but apparently is reoxidized.

REFLECTIVITY Our invention will produce highly reflective surfaces onmill finish wrought aluminum alloys. As an example, an unbutfed millfinished aluminum sheet (C57S alloy) was divided up into specimensnumbered 1 to 3 and 4* to 6*. Specimen l was left in the mill finishedcondition. Specimen 2 in the mill finished condition was chemicallybrightened. Specimen 3 in the mill finished condition was chemicallybrightened and electro-polished. Specimens 4* through 6* were wirebrushed. Specimen 4* Table 1 Control Chem. B Combined Specimen Number '1S T S T S In addition to producing highly reflective surfaces on wroughtaluminum alloys our invention will also produce attractive surfaces onuntreated cast aluminum alloys. In these cases, however, since theoriginal surface roughness is considerably greater in addition to beingmore porous, than on wrought products, the final appearance is generallyone of high diffuse reflectivity rather than specular reflectivity.

Having described our invention, we claim:

1. A method for surface brightening and polishing an article composed ofa metal selected from a group consisting of aluminum and its alloyscomprising: providing a bath consisting essentially of sulfuric acid, pm

acid and water, corresponding, in nature and amount, at

room temperature to a mixture of commercial sulfuric and phosphoricacids in room temperature volume ratios ranging from 70% commercialsulfuric over 30% commercial phosphoric to commercial sulfuric over 15%commercial phosphoric so that the total water-free acid content of thebath ranges from about 88%% to about 90 /z% of the total volume of thebath; heating the bath to a temperature ran ing from about 2m to about300 F.; and dipping the article into the heated bath for surfacebrightenin purposes.

2. The method of claim 1 wherein: the bath is heated to a temperaturenot substantially lower than 230 F.; and the dipping step includesdipping the article into the bath, without an electric current flowingthrough the bath, for chemical brightening purposes.

3. The method of claim 1 wherein: the bath is heated to a temperaturenot substantially higher than 250 F.; and the dipping step includesdipping the article into the bath, with an electric current flowing fromthe article through the bath, for electrolytic polishing purposes.

4. The method of claim 1 wherein: the bath temperature ranges from about230 F. to about 250 F.; and the dipping step includes dipping thearticle into the bath without an electric current flowing through thebath for chemical brightening purposes and thereafter causing anelectric current to flow from the article through the bath forelectrolytic polishing purposes.

5. The method of claim 1 including: providing another bath substantiallyduplicating the first mentioned bath; maintaining one bath at atemperature not substantially below 230 F. and the other at atemperature not sub stantially above 250 F.; and thereafter performingthe dipping step by dipping the article into said one bath without anelectric current flow therethrough for chem ical brightening purposesand into said other bath with an electric current flowing from thearticle through the bath for electrolytic polishing purposes.

6. The method of claim 1 wherein: the providing step includes providinga bath having, as another component, a relatively small amount of asilver salt.

7. The method of claim 6 wherein: the bath is heated to a temperaturenot substantially below 230 F.; and the dipping step includes dippingthe article into the bath without an electrical current flowing throughthe bath for chemical brightening purposes.

8. The method of claim 6 wherein: the bath is heated to a temperaturenot substantially above 250 F.; and the dipping step includes dippingthe article into the bath with an electric current flowing from thearticle through the bath for electrolytic polishing purposes.

9. The method of claim 6 wherein: the bath is heated between about 230F. and 250 F.; and the dipping step includes dipping the article intothe bath without an electric current flowing through the bath forchemical brightening purposes and thereafter causing an electric currentto flow from the article through the bath for electrolytic polishingpurposes.

10. The method of claim 6 including: providing another bathsubstantially duplicating the first mentioned bath; maintaining one bathat a temperature not substantially below 230 F. and the other notsubstantially above 250 F.; and performing the clipping step by dippingthe article into said one bath without an electric current flow thereinand into said other bath with an 7 electric current flowing therein fromthe article through said other bath.

11. A composition for use at an elevated temperature in surfacebrightening and polishing an article composed of a metal selected from agroup consisting of aluminum and its alloys comprising: a bathconsisting essentially of sulfuric acid, a material selected from thegroup consisting of phosphoric acid and acid salts thereof, and water,corresponding in nature and amount, at room temperature and on the basisof equivalent phosphoric acid, to a mixture of commercial sulfuric andphosphoric acids in room temperature volume ratios ranging from 70%commercial sulfuric over 30% commercial phosphoric to 85% commercialsulfuric over 15% commercial phosphoric so that the total water-freeacid content of the 8 bath ranges from about 88 /2% to about 90V2% ofthe total volume of the bath.

12. The composition of claim 11 including: a relatively small amount ofa silver salt.

References Cited in the file of this patent UNITED STATES PATENTS2,650,156 Shelton-Jones Aug. 25, 1953 FOREIGN PATENTS 798,721 FranceMar. 11, 1936 638,321 Great Britain June 7, 1950 675,444 Great BritainJuly 9, 1952 679,078 Great Britain Sept. 10, 1952

1. A METHOD FOR SURFACE BRIGHTENING AND POLISHING AN ARTICLE COMPOSED OFA METAL SELECTED FROM A GROUP CONSISTING OF ALUMINUM AND ITS ALLOYSCOMPRISING: PROVIDING A BATH CONSISTING ESSENTIALLY OF SULFURIC ACID,PHOSPHORIC ACID AND WATER, CORRESPONDING, IN NATURE AND AMOUNT, AT ROOMTEMPERATURE TO A MIXTURE OF COMMERCIAL SULFURIC AND PHOSPHORIC ACIDS INROOM TEMPERATURE VOLUME RATIOS RANGING FROM 70% COMMERCIAL SULFURIC OVER30% COMMERCIAL PHOSPHORIC TO 85% COMMERCIAL SULFURIC OVER 15% COMMERCIALPHOSPHORIC SO THAT THE TOTAL WATER-FREE ACID CONTENT OF THE BATH RANGESFROM ABOUT 88 1/2% TO ABOUT 90 1/2% OF THE TOTAL VOLUME OF THE BATH,HEATING THE BATH TO A TEMPERATURE RANGING FROM ABOUT 200*F. TO ABOUT300*F., AND DIPPING THE ARTICLE IN THE HEATED BATH FOR SURFACEBRIGHTENING PURPOSES.
 3. THE METHOD OF CLAIM 1 WHEREIN: THE BATH ISHEATED TO A TEMPERATURE NOT SUBSTANTIALLY HIGHER THAN 250*F., AND THEDIPPING STEP INCLUDES DIPPING THE ARTICLE INTO THE BATH, WITH ANELECTRIC CURRENT FLOWING FROM THE ARTICLE THROUGH THE BATH, FORELECTROLYTIC POLISHING PURPOSES.